In recent years, large and thin television receivers and back-projection projectors have become widespread in order to obtain stronger visuals within a setting space available in a home.
These television receivers and back-projection projectors have been becoming much thinner than conventional models with technical advances. Also, conventional cathode ray tubes (CRTs) are being replaced by display devices using liquid crystal or flat display panels such as plasma display panels (PDPs). In a flat display panel, a method in which a display to be hung on a wall and a television tuner unit are separated and are connected by using a cable has been proposed.
Further, digital versatile discs (DVDs) for high definition television (HDTV) have been proposed.
In DVDs or the like for HDTV, however, digital-signal processing is performed. In a presently-adopted connection method such as a digital terminal using an analog signal, a digital signal is once converted to an analog signal and then the analog signal is converted to a digital signal in a display side, so as to drive a display device, such as a display unit in HDTV (for example, see Japanese Unexamined Patent Application Publication No. 2001-36723). By performing the digital (D)/analog (A) conversion and the A/D conversion, unfavorable signal deterioration occurs.
In order to overcome this problem, another technique has been proposed. That is, a connection method using a digital signal, which has become the mainstream for connecting a personal computer (PC) and a liquid crystal display, is adopted for consumer apparatuses such as television receivers.
In the digital-signal connection, however, if data is duplicated in a state of digital signal, duplicates of expensive movie software or the like can be easily produced such that the duplicates include no degradation in the picture quality. Therefore, in order to protect a copyright, an encrypting process must be performed on a video signal at a connector portion for realizing the digital-signal connection and a video signal superimposed by a voice signal.
At the beginning of the encrypting process, a general authentication process is performed. That is, for example, each of transmitter and receiver sides holds several tens of groups of secret numeric sequences of several tens of bits, which serve as a common secret encryption key. An arbitrary about half of the numeric sequences are selected by a public key, so that a new random number sequence is generated. Then, the transmitter examines the numeric sequences so as to check whether or not the receiver on the other side is authorized to receive a signal to be transmitted. Accordingly, the transmitter judges that the receiver is formally authorized.
Then, the transmitter circulates the numeric sequences in a random-number generating circuit by using horizontal and vertical synchronizing signals serving as a reference for synchronizing a video signal, and randomly inverts the digital video signal by using this random number sequence, so as to encrypt the digital video signal, which is then transmitted. The receiver side generates a random number sequence by using the same numeric sequences and inverts the video signal so as to decrypt the encrypted signal, and then displays the original correct video signal.
At this time, each of the transmitter and receiver sides generates an encryption key based on the same numeric sequence. Then, by using this encryption key, each side cyclic-generates a random number sequence for several tens of clocks of a vertical synchronizing signal period defined by a pixel clock of the video signal, and once stores the value as a frame-key value. Then, the random number sequence is circulated and stopped in the same way in several tens of clocks of a horizontal synchronizing signal period of each video line which is subsequently input. Then, the value is led to a cyclic shift register circuit in the subsequent stage, and is generated as an encrypting random number sequence by a pixel clock of a display period of an effective video signal.
Therefore, even if the starting point of a display period of an effective video signal slightly varies due to noise, such as electro-static discharge (ESD), and accordingly an error is caused, a random number sequence generated in the immediately preceding horizontal synchronizing signal period is used in random-number generation for the next line. Therefore, continuous disturbance in decryption can be prevented.
If noise is mixed into a horizontal synchronizing signal or if a dropout of the horizontal synchronizing signal is caused, numeric sequences for generating random numbers may proceed or delay by one line. In this case, too, a random number sequence is generated by using numeric sequences stored in the immediately preceding vertical synchronizing signal period in the next frame (next field in a case of an interlace signal). Therefore, although a part corresponding to several lines may be distorted in the upper side of a screen, distortion of the displayed picture, which is caused by abnormal decryption, can be minimized to within one frame (or one field).
However, if noise is mixed into a vertical synchronizing signal or if a dropout of the vertical synchronizing signal is caused, a once-stored random number sequence serving as a frame-key value goes out of synchronization. Accordingly, distortion of the displayed picture caused by a decryption error continues for a long time. In general, however, the transmitter side constantly examines a secret value used as a reference value for generating random numbers in a cycle of about 128 frames so as to check whether a connected apparatus in the receiver side is continuously valid, and resets the reference value for generating random numbers. Therefore, a decryption error can be suppressed to within about two seconds (128 frames) at the maximum.
In scrambled digital television broadcasting or the like in the MPEG (Moving Picture Experts Group) 2 standard, a compressed video signal corresponding to several tens of frames is captured and then decoding and descrambling are performed at channel switching or the like. Thus, a pictureless-and-silent state may continue for about two seconds. If encryption and decryption errors are added thereto, the pictureless-and-silent state or a picture of encrypted random numbers (picture of a noise signal) continues for four seconds at the maximum, which is undesired.
Also, in digital broadcasting, particularly in commercials, a so-called “mixed-mode broadcasting”, in which high-resolution-signal broadcasting and ordinary-signal broadcasting are switched to/from each other or a signal is switched in the opposite direction, may be performed. In this case, too, the “mixed-mode broadcasting” cannot be displayed if decryption of encrypted data delays.
Likewise, when a wall-hung television set is adopted, a cable used for digital connection needs to have a length of about 5 m. In this case, if an undesired noise component is mixed into a vertical synchronizing signal due to a momentary external noise, such as ESD, or if poor contact is caused at a cable connector portion, an undesired state where encrypted data cannot be decrypted continues for about two seconds.